Apparatus and processes for the measurement of exhaust gas components of internal combustion engines have been known for a long time. In the so-called CVS (Constant Volume Sampling) process, the exhaust gas produced during a test is introduced into the intake flow of a constantly transporting conveyor. The conveyor has a transport capacity, which lies clearly above the maximum exhaust gas volume flow of the vehicle engine under full load. The volume difference that occurs between the exhaust gas quantity and the conveyor capacity is equilibrated by intake of filtered fresh air that fulfills the dual functions of providing exhaust gas dilution and constant gas/air volume flow rate. Samples are taken and collected in sample bags from the exhaust gas air mixture during the entire test with uniform volume flow. The concentrations of pollutants and carbon dioxide are measured in the samples prepared in this way. However, a problem arises in that the emissions of modern engines and vehicles are becoming increasingly smaller. The measured concentrations often approach the detection limits of the analytical technique, so that the results of analysis are unreliable. One remedy is to make the dilution ratio smaller and thus the concentration of exhaust gas in the bags will be higher. This procedure is limited by the water concentration that also increases. Condensation of water in the exhaust bags and in the lines leading therein as well as in the analysis lines, however, must be avoided, since this leads to the falsification of the measurement results (several pollutants are soluble in water and also precipitate therein). This problem is eliminated at the present time by heating the exhaust gas sample bag, sample lines leading thereto from the CVS, as well as the analysis lines.
In heating the sample gas, there is now the problem that in order to avoid condensate, the entire device including the exhaust bag, the sampling lines and the analysis lines must be heated. This extensive heating and temperature regulation plus monitoring are time-consuming and expensive. Since any cold spots in the piping system must be avoided, all valves, for example, must also be at an elevated temperature, which requires an increased expenditure for insulation. The operating costs are also considerable, due to the high consumption of electrical power involved in the heating. In addition, there is no flexibility in the construction of the complete test stand.